1) Field of the Invention
The present invention relates to a liquid crystal layer having a satisfactory voltage holding ratio.
2) Description of the Related Art
Conventionally, in a field of image display devices, an image display device that uses a change in alignment of liquid crystal molecules (hereinafter, referred to as “liquid crystal display”) has been known. The liquid crystal display includes a liquid crystal display cell having a liquid crystal layer sealed into between two glass substrates arranged in an opposed manner. The liquid crystal display displays an image in such a manner that a predetermined electric field is applied to the liquid crystal molecules forming the liquid crystal layer to change the alignment of the liquid crystal molecules and the light transmittance of the liquid crystal layer.
Specifically, in the conventional liquid crystal display, a pixel electrode is provided on a glass substrate correspondingly to a display pixel, and a switching element including thin film transistor (TFT) or the like is arranged on each of the pixel electrodes. The pixel electrode has a storage possible structure such that a common electrode arranged on the opposed glass substrates and a liquid crystal layer sandwiched between both the electrodes form a capacitor. When a current flown in from outside via the switching element stores charges on the capacitor, a predetermined electric field is applied to the liquid crystal molecules so that the alignment of the liquid crystal molecules changes and the light transmittance changes, thereby enabling image display. The charges stored on both the electrodes are held even after the switching element is turned OFF, and therefore a state such that the alignment of the liquid crystal molecules has been changed is maintained until the switching element is again turned ON so as to collect the charges, thereby maintaining the displayed image.
With the above structure, the liquid crystal display has excellent characteristics such as space saving, low voltage operation, and low power consumption due to its thinner structure as compared to those of a CRT display or the like using a cathode-ray tube. The liquid crystal display is becoming remarkably widespread as an image display device in a personal computer, a personal digital assistant, and the like.
In order to maintain a displayed image, a liquid crystal material for the liquid crystal display needs to have a high voltage holding ratio. The reason for this is that when the liquid crystal material has electrical conductivity, a current flows between electrodes forming a capacitor, and an electric field intensity applied to a liquid crystal layer is weakened. Therefore, the alignment of liquid crystal molecules is disordered, which interferes with image display.
In general, it is known that the liquid crystal molecule itself does not have the electrical conductivity, an impurity or the like mixed with the liquid crystal material resolves to produce ions. The produced ions function as a conductive carrier in the liquid crystal material so that the electrical conductivity is generated between the electrodes, thereby lowering the voltage holding ratio. Therefore, how to lower concentration of the impurity in the liquid crystal material has been studied in order to realize the high voltage holding ratio.
Conventionally, to increase the voltage holding ratio, a fluorine-based nematic liquid crystal material is adopted and a cleanliness of a manufacturing line is improved. The reason why the fluorine-based nematic liquid crystal material is adopted is that this material has an advantage that impurity solubility of this material is much smaller than that in another liquid crystal materials. The mixed impurity resolves in the liquid crystal material to be ionized, and thereby the liquid crystal material exhibits the electrical conductivity. Therefore, low impurity solubility makes it possible to decrease the electrical conductivity, thereby improving the voltage holding ratio. By improving the cleanliness of the manufacturing line, a quantity of the impurities to be mixed can be suppressed, thereby improving the voltage holding ratio.
However, the improvement in the voltage holding ratio based on the conventional technology still has a problem. Firstly, the concentration of the impurity is suppressed to a value not more than predetermined concentration at the time of manufacturing based on the conventional technology. Further, even if electrical conduction in liquid crystal can be suppressed, there still remains a problem that the voltage holding ratio is lowered according to long-time use. This is because it is considered that an impurity of an uncured component of a material around the liquid crystal layer, a metal ion included in moisture intruding into the liquid crystal layer, or the like gradually intrudes inside the liquid crystal layer. Therefore, even if the liquid crystal material has the high voltage holding ratio just after the manufacturing, the voltage holding ratio is gradually lowered, and thus a problem such that unevenness occurs in a displayed image is elicited.
There is another problem such that a degree of freedom in relation to a selection of liquid crystal materials is limited. For example, in order to realize low-operating voltage and high-speed response, the use of a liquid crystal material having high dielectric constant anisotropy is proposed. However, the liquid crystal material having high dielectric constant anisotropy that is known so far, has high impurity solubility, and therefore dissolution of the impurity cannot be suppressed unlike the above case. Consequently, when the liquid crystal material having the high dielectric constant anisotropy is used in the liquid crystal display, it is necessary to use a method different from any conventional method in order to improve the voltage holding ratio.
There is still another problem that cleaning of the manufacturing line is not easy in actual cases. Since the mixing of the impurity into the liquid crystal material occurs mainly when the liquid crystal material is manufactured and is filled into a cell, it is necessary to clean the manufacturing line at these steps to the limit. In order to achieve this, for example, it is necessary to frequently clean a filling apparatus used for filling the liquid crystal material into the cell. This makes the manufacturing steps complicated, and thus the manufacturing cost rises.
As explained above, according to the conventional method, it is not easy to suppress the mixing of the impurity into the liquid crystal material, and it is difficult to keep the high voltage holding ratio. It is, therefore, necessary to keep the high voltage holding ratio based on a viewpoint other than that the concentration of the impurity is lowered.